Due to the time required to heat up the main aftertreatment devices to achieve catalytic light-off, NOx emissions from engine-out cold-starts add up to a significant fraction of total NOx emissions.
The inventors herein have recognized that an exhaust system, such as a branched exhaust system, with first and second turbines that includes an emission-control device containing a zeolite or similar adsorbent may be used to reduce NOx emissions during non-warmed exhaust gas conditions while providing improved boost control during engine operation.
In one example approach, a method for an exhaust system having a first turbine and a second turbine, comprises: during a first duration when exhaust temperature is below a first temperature threshold, directing exhaust gas through the second turbine and an emission-control device, and adjusting the second turbine to control intake boost; and during a second duration following the first, directing exhaust gas through the first turbine, and adjusting the first turbine to control intake boost. In one example, the emission-control device may be purged when exhaust temperature is above a second temperature threshold higher than the first temperature threshold, where during the second duration exhaust gas does not flow through the emission-control device.
In this way, during non-warmed exhaust conditions, NOx emissions may be directed through an emission-control device containing, for example, a zeolite. NOx may be adsorbed by the emission-control device while the exhaust is heated. The adsorbed NOx may then be substantially stored in the emission-control device until a NOx reducing device, e.g., an ammonia-catalyzed selective catalytic reducer, has been sufficiently heated to become catalytically active. Postponing the release of the stored NOx to a NOx reducing device in this way may decrease NOx emissions since a greater portion of the NOx emitted by the engine during non-warmed exhaust conditions is reduced.
Furthermore, in such an approach, the first and second turbines may provide a greater degree of boost control in order to reduce boost fluctuations while transitioning between different operating modes of the branched exhaust system. Furthermore, the turbines may be advantageously used to accommodate transient torque requests, and maintain boost at desired levels, for example, through coordinated control of the branched exhaust system during warmed-up operating conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.